I cant seem to find any thing on the Mercury pistons that have the expansion slot in them. I need to know when you install them in the engine does the slot face the Thrust side of the cylinder wall or towards the intake. The engine I took apart was 50/50 2 one way 2 the other way. Those 4 pistons were burnt with a hole on the thrust side of the pistons. I would think the piston slots would need to be all one way not random. I need a Flat head expert to answer this. I have rebuilt lots of Ford engines but never a Mercury with these pistons ,